SYNTAX: thematic roles are semantic roles in a system which takes location and movement through space as (some of) the key notion in the interpretation of natural language sentences.
EXAMPLE: the sentence Don gives Ed a donut is about a relocation or transfer of a donut (the Theme) from Don (the Source) to Ed (the Goal). In syntactic theory thematic roles are usually referred to as Theta-roles with not much concern for their content. See Thematic theory.